Metal-ion secondary batteries that have solid electrolyte layers using flame-retardant solid electrolytes (for example, a lithium-ion secondary battery. Hereinafter they may be referred to as “all-solid-state batteries”.) have advantages such as an easy simplification of systems for securing safety.
Sulfide solid electrolytes of high Li-ion conductivity are known as solid electrolytes used for all-solid-state batteries. Examples of known sulfide solid electrolytes include Li2S—P2S5 based electrolytes, Li2S—P2S5—LiBr—LiI based electrolytes that are obtained by adding LiBr and LiI to Li2S—P2S5 based electrolytes, and Li2S—P2S5 based glass ceramics and Li2S—P2S5—LiBr—LiI based glass ceramics which are glass ceramics thereof.
A problem with sulfide solid electrolytes is that elemental sulfur (hereinafter may be simply referred to as “elemental S”) is easy to mix as an impurity. The following (1) to (4) are considered to be factors in mixing elemental S into sulfide solid electrolytes:
(1) Sulfide that is to be used as raw material for a sulfide solid electrolyte deteriorates while stored, and part thereof changes to an impurity (for example, P2S5 changes to P4S9 and P4S7). This impurity has a composition of fewer S atoms than sulfide before the change, and thus elemental S forms as a by-product;
(2) if raw material contains elemental S according to (1), this elemental S cannot be in contact with other kinds of raw material, which brings low reactivity, and many residues are left even after electrolytes are synthesized;
(3) elemental S forms while sulfide solid electrolytes are synthesized; and
(4) S—S bonds form, to form elemental S in a heat-treating step for making sulfide solid electrolytes, glass ceramics.
For example, Patent Literature 1 discloses that capacity of a battery is prevented from decreasing, by 1 wt % or less of an elemental sulfur component in a sulfide solid electrolyte that is synthesized from at least Li2S, and one or more sulfide(s) selected from P2S3, P2S5, SiS2, GeS2, B2S3, and Al2S3 as a technique of reducing an elemental sulfur component existing in a sulfide solid electrolyte, and discloses that raw material, or a sulfide solid electrolyte produced by raw material is washed with an organic solvent as a method of removing elemental sulfur.
Patent Literature 2 discloses that crystallinity of crystalline ion conductive material is decreased by mechanical milling, and the ion conductive material of decreased crystallinity is heated as a method of producing ion conductive material of a LGPS structure, and discloses vibrating milling as the mechanical milling.